The goal of this project is to analyze the organization, in terms of evolutionary and maturational development, of the visual system of vertebrates, with particular reference to midbrain mechanisms. Electrophysiological studies are planned to reveal specialization of neuronal response to visual stimuli for cells in the thalamic pulvinar and posterior groups that receive major input from the midbrain, particularly the superior colliculus and pretectal nuclei. Morphological studies planned deal with the major areas of uncertainty concerning midbrain structures and their thalamic projections. The laminar distribution of collicular neurons and their axonal trajectories are being studied by axonal protein transport using radioactive markers, including those capable of revealing postsynaptic labelling. Some limited experience with both retrograde and anterograde axonal transport of horseradish peroxidase is also in progress. BIBLIOGRAPHIC REFERENCES: Stein, B.E., Magalhaes-Castro, B., and Kruger, L. Superior colliculus visuotopic-somatotopic overlap. Science 189: 224-226 (1975). Stein, B. E., Magalhaes-Castro, B. and Kruger, L. Relationship between visual and tactile representations in cat superior colliculus. J. Neurophysiol. 39: 401-419 (1976).